1. Field of the Invention
This invention relates to radio communication systems and methods, and more particularly, relates to interference canceling systems and methods for minimizing or eliminating interference in radio receivers due to unwanted signals. Even more specifically, this invention relates to a cancellation system capable of suppressing interference when conventional techniques are ineffective and access to interfering sources or multiple reference couplers and their associated cables are impractical because of complexity and weight.
2. Description of the Prior Art
Receiver operation within a radio communication system may be disrupted by interfering signals generated by collocated transmitters or remote transmitters radiating within the receiver passband. These interfering signals are received by a system receiving antenna and are present on a system receive line. A collocated source may interfere with the receiver due to the finite isolation between the transmit and receiving antennas. Separating the transmitter and receiver in order to increase the isolation is often times impossible due to physical constraints, such as in the case of an airborne platform. Interfering signals received from a remote transmitter at the receiving antenna which are stronger than the signal of interest will prevent the signal of interest from being detected. Conventional techniques such as filtering to remove the interfering signals when they are not closely distinguished in frequency from the signal of interest place unacceptable limitations on system frequency response. Interference cancellation systems solve such problems associated with both the remote and collocated transmitters.
An interference cancellation system takes a sample of an interfering signal for use as a sample reference signal and synchronously detects the presence, amplitude and phase of an interfering (undesired) signal on the system receive line. The system generates a cancellation signal which is adjusted in magnitude and phase such that the result is equal in amplitude and 180.degree. out of phase with the interfering signal on the receive line. Theoretically, the vector sum of the two signals will cancel, leaving only the signal of interest as input to the receiver. A detailed explanation of the structure and workings of an interference cancellation system can be found in commonly owned U.S. Pat. No. 4,952,193 to Ashok Talwar, the disclosure of which is incorporated herein by reference.
FIG. 1 is a functional diagram of a conventional "N-channel" interference cancellation system 1 coupled to a radio receiver system, shown generally as including a receiving antenna 2, a receiver 4 and a receive transmission line 5, interconnecting the receiving antenna 2 and the receiver 4. The interference cancellation system is also coupled to three of N transmitters, not associated with the radio receiver system. Three transmitters 16, 18, 19, each output an energy signal for transmission by three associated antennas 8, 10, 12, electrically connected by antenna transmission lines 26, 28, 30, respectively. The energy signals transmitted from antennas 8, 10, 12 may be received by receiving antenna 2 and interfere with the operation of receiver 4. The interference cancellation system 1 samples the signals output by transmitters 16, 18, 19, thereby providing reference signals for use in canceling the interfering signals present on the receive transmission line 5.
A first, second and third coupler 20, 22, 24 (of N couplers) are coupled to antenna transmission lines 26, 28, 30, respectively. The couplers sample the signals, i.e., interfering signals, generated by transmitters 16, 18, 19, respectively, and generate corresponding reference signals on their outputs. Three signal controllers 32, 34, 36 are shown electrically connected to couplers 20, 22, 24, respectively, through transmission lines 21, 23, 25, respectively, for receiving the reference signals. Three synchronous detectors 38, 40, 42 are shown electrically connected to couplers 31, 33, 35, respectively. The couplers 31, 33, 35 sample the reference signals present on transmission lines 21, 23, 25, thereby providing a portion of the reference signal to the synchronous detectors.
A sampler coupler 44 samples the interfering signals present on receive transmission line 5. A power divider 45 provides a portion of the sampled interfering signals to each of synchronous detectors 38, 40, 42. Synchronous detectors 38, 40, 42 then compare the amplitude and phase of the sampled interfering signals present on the receive line 5 with the amplitude and phase of the reference signals sampled by couplers 20, 22, 24 (corresponding to transmitters 16, 18, 19). Based on the comparison, control signals are generated and output to signal controllers, 32, 34, 36.
Upon receipt of the control signals, signal controllers 32, 34, 36 generate cancellation signals which correspond, in effect, to the reference signals adjusted in phase and amplitude. The cancellation signals are injected into the receive line 5 via a power combiner 47 and summer coupler 46 in order to cancel or suppress the undesired interfering signals present there. Signal controllers 32, 34, 36 are continuously driven by the control signals from the synchronous detectors 38, 40, 42, thereby defining an adaptive control loop for automatically adjusting the amplitude and phase of the reference signals and generating corresponding cancellation signals. The reference signals used by the interference cancellation system for interference suppression are therefore derived from the couplers installed in the transmit line of each of the interfering sources (or from an auxiliary antenna in the case of a remote transmitter).
The conventional "N-channel" interference cancellation system shown in FIG. 1 requires an adaptive control loop for cancelling the interference generated by each interfering source. Each adaptive control loop requires a separate coupler to sample each interfering source. Multiple couplers and adaptive control loops may impose unacceptable weight and complexity requirements for many applications. For example, an airborne platform having N interfering on-board sources, such as switching power supplies, computer buses, etc., collocated with a receiving antenna would require at least a coupler, detector and signal controller for each interfering source. The added elements corresponding to each interfering source burden the airborne platform with additional weight.
Problems of complexity result from the need to coordinate the multiple adaptive loops required for multiple interfering sources. For example, a cancellation signal corresponding to each source of interference must be injected into the receive line. The amplitude, phase and time match between the adjusted reference signal (i.e., the cancellation signal) and the interfering signal present on the receive line must be such that the interfering signal present on the receive line is canceled. In addition to problems of complexity, often times a receiver is so remote from an operating station that access to the receiver is impossible thereby rendering it functionally impossible to derive an reference signal.